The present invention relates to an optical cable with an expandable fiber capacity. More particularly, the present invention relates to an expandable optical cable having conduits capable of receiving additional optical fibers as needs arise after the cable is installed.
Typically, optical fiber cables are installed underground and, in many situations, in ducts of an optical-cable passageway. When installed in ducts, the cables may be pulled through the ducts or blown through them. In a pulled installation, a leading end of a cable is attached to a line and pulled through the duct. In an air-blown installation, a gas is flowed into an end of the duct along with the cable and in a direction in which the cable is installed. In this manner, frictional forces produced between the flowing gas and the cable help to move the cable through the duct. The length of cable that can be blown through a duct can be increased by applying an additional pulling force or by a parachute-like device attached to the leading end of the cable. Unlike pulled cables, however, air-blown cables do not have to withstand high tensile loads, and, consequently, their structure can be less substantial. Air-blown cables do require a degree of stiffness, though, to facilitate their movement through ducts.
When installing optical fiber cables, it is often desirable to provide for future growth in demand along a particular communication route. An optical fiber cable installed today may not provide sufficient capacity to meet tomorrow's needs. Currently, future demand is typically accommodated by including additional ducts in a cable assembly along the particular route that are unused initially. The additional ducts meet future demand by providing a pathway through which an additional cable can be installed at some point in the future. This method of installing a separate duct and cable along a route, however, has a few disadvantages. For example, two different products (a duct and a cable) are required which often come from two different manufacturers. Additionally, installation time is increased as the two separate components necessitate additional handling.
A second option for addressing future demand is to install a cable with greater capacity than that required at the time of installation. In particular, a cable containing more optical fibers than is immediately necessary can be installed. But this is an expensive alternative as the capital investment for predicted future growth must be made at the time of initial installation. Deferring capital investment until the time of expansion is generally more desirable.
A third option is disclosed in U.S. Pat. No. 6,101,304. The '304 patent describes a so-called air blown fiber tube assembly that combines several ducts into one unit and surrounds it with an outer jacket. In particular, the assembly has a central innerduct surrounded by a plurality of strength elements and air blown fiber tubes. The assembly is installed underground using conventional boring or direct burial methods. After installation, a conventional cable (optical fiber, power, etc.) may be pulled through the central innerduct of the tube assembly. As well, fiber assemblies may be air blown into the air blown fiber tubes.
PCT application GB01/GB03343, having a priority date of Aug. 7, 2000, discloses an expandable optical fiber cable having a tubular central strength member. In this PCT application, the cable has optical fibers in buffer tubes that are stranded around the central strength member. Additional optical fibers may be blown into the tubular central strength member. PCT application GB01/GB03343 and the present application were, at the time the present invention was made, owned by the same entity or subject to an obligation of assignment to the same entity.
Applicants have found that the proposals in the known art do not provide sufficient capacity for expansion while minimizing installation costs. For example, the tube assembly of the '304 patent requires that installation take place in two steps before any operation can occur. First, the tube assembly must be installed underground, then the cables must be installed by pulling and blowing into the assembly conduits. Applicants have also noted that this tube assembly is impractical for upgrading a communication route that already has ducts installed underground.